Semiconductor thin film, thin film transistor, method for manufacturing same, and manufacturing equipment of semiconductor thin film

ABSTRACT

A method for manufacturing a semiconductor thin film is provided which can form its crystal grains having a uniform direction of crystal growth and being large in size and a manufacturing equipment using the above method, and a method for manufacturing a thin film transistor. In the above method, by applying an energy beam partially intercepted by a light shielding element, melt and re-crystallization occur with a light-shielded region as a starting point. The irradiation of the beam gives energy to the light-shielded region of the silicon thin film so that melt and re-crystallization occur with the light-shielded region as the starting point and so that a local temperature gradient in the light-shielded region is made to be 1200° C./μm or more. In the manufacturing method, a resolution of an optical system used to apply the energy beam is preferably 4 μm or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of application Ser. No. 10/838,368,filed May 5, 2004, which claims priority to Japanese Patent ApplicationNo. 2003/131405 filed May 9, 2003, entitled SEMICONDUCTOR THIN FILM,THIN FILM, TRANSISTOR, METHOD FOR MANUFACTURING SAME, AND MANUFACTURINGEQUIPMENT OF SEMICONDUCTOR THIN FILM, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing asemiconductor thin film making up a semiconductor device, such as memoryand CPU (Central Processing Unit), a thin film transistor (TFT) made upof a same semiconductor thin film, and a method for manufacturing thesame semiconductor thin film and the same TFT, and a manufacturingequipment being used in manufacturing the same semiconductor thin film.

2. Description of the Related Art

A conventional product manufactured by forming a semiconductorintegrated circuit on a glass substrate is typified by a TFT. Aconventional typical TFT is constructed, as shown in FIG. 17, by forminga channel region 103, a source region 104, a drain region 105, and anLDD (Lightly-doped drain) region 106 on a glass substrate 101 coatedwith a substrate coating layer 102, and then by forming a gate electrode108 with a gate insulating film 107 being interposed between the gateelectrode 108 and such the regions 103, 104, 105, and 106 as above, andby forming a contact hole in deposited silicon dioxide 109, and furtherby being wired using a metal 110.

A TFT being generally and presently used is classified depending on itsactive layer and a hydrogenated amorphous silicon TFT andpoly-crystalline silicon TFT have come into wide use. A maximumtemperature being employed in a process of fabricating the hydrogenatedamorphous silicon TFT is about 300° C., which has achieved carriermobility of about 1 cm²/Vsec. On the other hand, in the case of thepoly-crystalline silicon TFT, by using, for example, a quartz substrateand by performing a high-temperature process of about 1000° C., apoly-crystalline silicon thin film having crystal grains being large insize is formed in which carrier mobility of about 30 to 100 cm²/Vsec hasbeen achieved. However, the poly-crystalline silicon TFT hasadisadvantage. That is, since the high-temperature process of about1000° C. is performed when the poly-crystalline silicon is manufactured,a low-priced glass having a low-softening point cannot be used, unlikein the case of the hydrogenated amorphous silicon TFT.

To solve this problem, formation of a poly-crystalline thin film at lowtemperatures by using laser crystallization technology is being studiedand developed. Laser crystallization technology is disclosed in, forexample, Japanese Patent Publication No. Hei 7-118443 in which anamorphous silicon thin film (also being called an a-Si thin film)deposited on an amorphous substrate is crystallized by being irradiatedwith a short wavelength laser and which is applied to manufacturing of aTFT being excellent in a charge mobility characteristic. This lasercrystallization technology has an advantage in that, since thetechnology enables crystallization of an amorphous silicon withoutelevating a temperature of an entire substrate, formation of asemiconductor device and/or a semiconductor integrated circuit on such alarge-area substrate as a liquid crystal display or a like and such alow-priced substrate as glass or a like is made possible.

Moreover, a method is disclosed in Japanese Patent Application Laid-openNos. Hei 11-64883 and 2000-306859 in which a poly-crystalline siliconthin film (also called poly-Si thin film) having crystal grains beinglarger in size is formed to manufacture a semiconductor thin film beingexcellent in a charge mobility characteristic.

For example, in the method disclosed in the Japanese Patent ApplicationLaid-open No. Hei 11-64883, an amorphous silicon thin film is irradiatedwith an excimer laser beam to melt and re-crystallize it and to form asilicon crystal having grains being large in size. In the methoddisclosed in the Japanese Patent Application Laid-open No. 2000-306859,by irradiating an energy beam sequentially in a manner in which alocation to be irradiated with the energy beam is shifted, apoly-crystalline semiconductor thin film is grown, that is, morespecifically, while a laser is irradiated two or more times in ascanning manner in a region where the amorphous silicon thin film ismelted and re-crystallized, the location to be irradiated with the laseris shifted in order to form a silicon thin film having crystal grainsbeing large in size. In each embodiment disclosed in the Japanese PatentApplication Laid-open No. Hei-64883, an aperture width (1 μm to 2 μm) issmaller than a width of a light-shielding mask pattern (1.5 μm to 5 μm)and an-energy beam is irradiated through the aperture having the smallwidth to melt the amorphous silicon thin film and the location to beirradiated with the laser is shifted to form a silicon thin film havingcrystal grains being large in size.

Furthermore, in the invention disclosed in the Japanese PatentApplication Laid-open No. 2000-306859, as in the case of the inventiondisclosed in the Japanese Patent Application Laid-open No. Hei 11-64883,the amorphous silicon thin film is repeatedly irradiated with a laserwhile the region to be irradiated with the laser is shifted, little bylittle (for example, by 1 μm), within a range in which the amorphoussilicon thin film is crystallized by one pulse irradiation with a laser.In the method employed in the above invention, a cyclical light and darkpattern in a light-shielded region is provided and a direction ofcrystallization is controlled by using a temperature gradient whichchanges depending on the light and dark pattern.

However, the technology disclosed in the Japanese Patent ApplicationLaid-open No. Hei 11-64883 has a problem. That is, in the manufacturingmethod disclosed in the Japanese Patent Application Laid-open No. Hei11-64883, the region where the amorphous silicon thin film is melt andre-crystallized is irradiated two or more times with a laser in ascanning manner and the location to be irradiated with the energy beamis shifted while being irradiated with the laser and, as a result, adifference between a width of a mask to intercept a laser and a width ofan aperture to let the laser be transmitted is small, which causes muchtime to be taken in forming a silicon thin film having crystal grainsbeing large in size.

Also, in the manufacturing method disclosed in the Japanese PatentApplication Laid-open No. 2000-306859, while the amorphous silicon thinfilm is crystallized, the regions to be irradiated with a laser aresequentially shifted so that the regions irradiated with the laser arenot overlapped to form the silicon thin film having crystal grains beinglarge in size. However, since its movement distance is as short as about1 μm, it takes much time to complete the processing in a specifiedregion.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a method for manufacturing a semiconductor thin film which usesan energy beam irradiating technique that can form crystal grains havinga uniform direction of growth of a crystal and being large in size and amanufacturing equipment using the above method. It is another object ofthe present invention to provide a method for manufacturing a TFT beingmade up of such the semiconductor thin film as manufactured by the abovemethod. It is still another object of the present invention to provide asemiconductor thin film manufactured by the above method and the TFTmanufactured by the above methods. It is still another object of thepresent invention to provide a manufacturing equipment of thesemiconductor thin film that can form crystal grains having a uniformdirection of growth of a crystal and being large in size.

According to a first aspect of the present invention, there is provideda method for manufacturing a semiconductor thin film including:

a step of causing a preformed semiconductor thin film to melt andre-crystallize with a light-shielded region therein as a starting pointof its melt and re-crystallization, by irradiating the preformedsemiconductor thin film with an energy beam partially intercepted by alight-shielding element;

wherein irradiation of the energy beam gives energy to thelight-shielded region so that melt and re-crystallization occur with thelight-shielded region as the starting point and so that a localtemperature gradient in the light-shielded region is made to be 300°C./μm or more.

Thus, according to the present invention, since the energy beam isapplied to the light-shielded region so that the local temperaturegradient in the light-shielded region in the semiconductor thin film ismade to be 300° C./μm or more, the temperature gradient serves as adriving force to make a crystal of the semiconductor thin film grow in aspecified direction. Moreover, since the energy beam is applied to thelight-shielded region in the semiconductor thin film to supply energy sothat the light-shielded region becomes a starting point for the melt andre-crystallization, it is made possible to make a crystal of thesemiconductor thin film grow by the above temperature gradient in aspecified direction with the light-shielded region as the startingpoint. As a result, since growth of a crystal occurs by the melt andre-crystallization from the starting point in a direction of thetemperature gradient, the semiconductor thin film whose crystal grainshaving a uniform direction of crystal growth and being large in size canbe formed effectively in a very short time.

In the foregoing, a preferable mode is one wherein a resolution of anoptical system used to irradiate the energy beam is 4 μm or less.

Thus, according to the present invention, since the resolution of theoptical system used to irradiate the energy beam is 4 μm or less, theabove temperature gradient in a size of 4 μm or less can be provided. Asa result, the temperature gradient serving as the driving force to makea crystal grow from the starting point in a specified direction can berealized.

Also, a preferable mode is one wherein the temperature gradient isprovided by an intensity gradient of an energy beam with 220 mJ/cm²/μmor more having reached the light-shielded region.

Thus, according to the present invention, the energy beam partiallyintercepted by the light-shielding element diffracts (turns around) thelight-shielded region to heat the semiconductor thin film and by theintensity gradient of the energy beam with 220 mJ/cm²/μm or more havingreached the light-shielded region, the above temperature gradientserving as the driving force to make a crystal grow in a specifieddirection is realized.

Also, a preferable mode is one wherein at least two-directionaltemperature gradient is provided in the light-shielded region.

Thus, according to the present invention, when the energy beam isirradiated in a manner in which it diffracts (turns around) thelight-shielded region, since at least two-directional temperaturegradient can be provided in the light-shielded region, a driving forceis given which causes the crystal to grow at least in two directionswith the light-shielded region as the starting point.

Also, a preferable mode is one wherein the light-shielding element is alight-shielding mask obtained by forming a light-shielding pattern on atransparent substrate.

Also, a preferable mode is one wherein, by one pulse applying an energybeam to the light-shielding elements on which the light-shieldingpatterns are periodically arranged to melt and re-crystallize an entiresurface of the semiconductor thin film.

Thus, according to the present invention, since the entire surface ofthe semiconductor thin film can be melted and re-crystallized by onepulse irradiation with the energy beam by using the light-shieldingelements on which the light-shielding patterns are periodicallyarranged, crystallization of a semi-conductor thin film whose crystalgrains having a uniform direction of the crystal growth and being largein size can be performed very effectively.

Also, a preferable mode is one wherein a ratio (P/L) between alight-shielding width L of the light-shielding pattern and a pitch P ofthe light-shielding pattern is 1 (one) or more.

Thus, according to the present invention, since larger crystal growthdriving force can be given by the above temperature gradient, even whenthe ratio (P/L) between the light-shielding width L of thelight-shielding pattern and the pitch P of the light-shielding patternis 1 (one) or more, a crystal being large enough to cover apertureportions among the light-shielding patterns can be grown. Even if theP/L ratio is as large as 10 or more, a crystal being large enough tocover the aperture portions among the light-shielding patterns can begrown, which is an effect that has not been acquired in the conventionaltechnology.

Also, a preferable mode is one wherein a light-shielding width of thelight-shielding pattern is 0.3 μm or more.

Thus, according to the present invention, since the local temperaturegradient in the light-shielded region is 300° C./μm or more, thelight-shielding width can be set at a lower limit value being as smallas 0.3 μm.

Also, a preferable mode is one wherein the semiconductor thin filmbefore being melted and re-crystallized is made from an amorphoussilicon or a poly-crystalline silicon.

Thus, according to the first aspect of the present invention, thesemiconductor thin film preformed before being melted andre-crystallized is made up of an amorphous silicon of which a meltingpoint is 1150° C.) or a poly-crystalline silicon of which a meltingpoint is 1410° C.

According to a second aspect of the present invention, there is provideda method for manufacturing a thin film transistor including:

a step of forming a crystallized film by making a crystal of asemiconductor thin film grow in one direction with a light-shieldedregion in the semiconductor thin film as a starting point by applying anenergy beam to the semiconductor thin film using a gate electrode formedwith a gate insulating film interposed between the gate electrode andthe semiconductor thin film as an light-shielding element;

wherein irradiation of the energy beam gives energy to thelight-shielded region so that melt and re-crystallization occur with thelight-shielded region as the starting point and so that a localtemperature gradient in the light-shielded region is made to be 300°C./μm or more.

Thus, according to the present invention, since, by using the gateelectrode formed with the gate insulating film interposed between thegate electrode and the semiconductor thin film, the energy beam isapplied to the light-shielded region so that the local temperaturegradient in the light-shielded region in the semiconductor thin film ismade to be 300° C./μm or more, the temperature gradient serves as adriving force to cause a crystal of the semiconductor thin film to growin a specified direction. Moreover, since the energy beam is applied tothe light-shielded region in the semiconductor thin film to supplyenergy so that the light-shielded region becomes a starting point forthe melt and re-crystallization, it is made possible to form acrystallized film obtained by making a crystal of the semiconductor thinfilm grow by the above temperature gradient in a specified directionwith the light-shielded region as the starting point. As a result, sincegrowth of a crystal occurs by the melt and re-crystallization from thestarting point in a direction of the temperature gradient, thesemiconductor thin film whose crystal grains having a uniform directionof the crystal growth and being large in size can be formed effectivelyin a very short time.

In the foregoing, a preferable mode is one wherein the temperaturegradient is provided by an intensity gradient of an energy beam with 220mJ/cm²/μm or more having reached the light-shielded region.

Thus, according to the present invention, the energy beam partiallyintercepted by the gate electrode serving as the light-shielding elementdiffracts (turns around) the light-shielded region to heat thesemiconductor thin film and by the intensity gradient of the energy beamwith 220 mJ/cm²/μm or more having reached the light-shielded region, theabove temperature gradient serving as the driving force to make acrystal grow in a specified direction is realized.

Also, a preferable mode is one wherein a width of the gate electrode is0.3 μm or more.

Also, a preferable mode is one wherein the semiconductor thin filmbefore being melted and re-crystallized is made from an amorphoussilicon or a poly-crystalline silicon.

Thus, according to the present invention, a melting point of theamorphous silicon is 1150° C. and a melting point of thepoly-crystalline silicon is 1410° C.

According to a third aspect of the present invention, there is provideda semiconductor thin film manufactured according to the method formanufacturing a semiconductor thin film stated in any one of Claim 1 toClaim 9,

wherein a thickness of the starting point portion from which growth ofthe crystal of the semiconductor thin film having been melted andre-crystallized starts is smaller than a thickness of a terminatingportion of growth of the crystal and growth of its crystal occurs in adirection of a thickness gradient.

In the foregoing, a preferable mode is one wherein a crystal of thesemiconductor thin film grows from its starting point for the growth, atleast, in two directions.

According to a fourth aspect of the present invention, there is provideda thin film transistor manufactured according to the method for a thinfilm transistor stated in Claim 10 to Claim 13, wherein a thickness ofthe starting point portion from which growth of the crystal of thesemiconductor thin film having been melted and re-crystallized whichmakes up the thin film transistor starts is smaller than a thickness ofa terminating portion of growth of the crystal and growth of its crystaloccurs in a direction of the thickness gradient.

According to a fifth aspect of the present invention, there is provideda manufacturing equipment of a semiconductor thin film including:

a irradiation device to cause, by applying an energy beam tolight-shielding elements each being arranged between a semiconductorthin film and an energy beam irradiating source, a crystal to grow in adesired direction with a light-shielded region of the semiconductor thinfilm as a starting point;

wherein the irradiation device has an optical system with a resolutionof 4 μm or less.

Thus, with the fifth aspect of the present invention, since themanufacturing equipment is equipped with the irradiation device havingthe optical system with a resolution of 4 μm or less, the localtemperature gradient which enables growth of the crystal to occur in adesired direction with the light-shielded region as the starting pointcan be given to the light-shielded region in which light is interceptedby the light-shielding element.

In the foregoing, a preferable mode is one wherein the light-shieldingelement is a light-shielding mask obtained by forming a light-shieldingpattern on a transparent substrate and a ratio (P/L) between alight-shielding width L of the light-shielding pattern and a pitch P ofthe light-shielding pattern is 1 (one) or more.

Also, a preferable mode is one wherein a light-shielding width of thelight-shielding pattern is 0.3 μm or more.

Furthermore, a preferable mode is one wherein the irradiation device hasa projection exposure unit which enables melt and re-crystallization tooccur on all surfaces of the semiconductor thin film by one pulseirradiation with an energy beam.

Thus, according to the present invention, since such the projectionexposure unit as above is provided, the semiconductor thin film whosecrystal grain shaving a uniform direction of crystal growth and beinglarge in size can be formed effectively in a very short time.

With the above configurations, unlike in the conventional case in whichseveral tens times to several hundreds times beam irradiation isrequired to re-crystallize a semiconductor thin film on a substratehaving even a big area size of, for example, 1 m square, by one pulseirradiation, re-crystallization of the semiconductor thin film on anentire surface of a substrate can be realized. As a result, a process ofirradiating a laser beam is speeded up several tens times to severalhundreds times and a maintenance life of the energy beam irradiationdevice can be made very long.

With another configurations, since a big crystal can be grown in aspecified direction, by matching a running direction of a carrier with adirection of growth of a crystal, unlike in the conventional method inwhich a channel is formed in a semiconductor thin film havingpolycrystalline grains arranged randomly, a high on-current with a highmobility can be obtained. As a result, an integrated circuit whosedriving voltage is lower than that of an integrated circuit made up ofthin film transistors formed on a glass substrate by the conventionalmethod and whose operating speed is high can be achieved and, when theintegrated circuit of the present invention is employed, for example, ina liquid crystal display, both a pixel TFT to drive each pixel and aperipheral driving circuit are simultaneously formed on a same glasssubstrate, which serves to enable manufacturing process costs to bereduced and peripheral driver integrated circuits to be miniaturized andenables creation of new electronic devices.

With still another configurations, since the irradiation device havingan optical device with high resolution is provided, a local temperaturegradient which enables growth of the crystal to occur in a desireddirection with a light-shielded region as a starting point can be givento the light-shielded region in which light is intercepted by thelight-shielding element. As a result, melt and re-crystallization of asemiconductor thin film can be very effectively performed. A remarkableeffect, in particular, is that melt and re-crystallization can beachieved by one pulse irradiation even if a semiconductor thin film hasa large area.

Additional effects are that, the semiconductor thin film an/or the TFTmanufactured by the methods of the present invention can be used infunctional devices such as a display, sensor, printing device, or a likeand in semiconductor devices such as a memory, CPU (Central ProcessingUnit), or a like and, in particular, in a TFT (thin film transistor),SOI (Silicon on Insulator) transistor, and inverter formed on aninsulator making up a semiconductor device or functional devices orelectronic devices using them.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages, and features of the presentinvention will be more apparent from the following description taken inconjunction with the accompanying drawings in which:

FIGS. 1A and 1B are diagrams explaining a method for manufacturing asemiconductor thin film according to a first embodiment of the presentinvention;

FIG. 2 shows a photograph of a magnified plan view of a growing state ofa silicon crystal observed when melt and re-crystallization haveoccurred by using a light-shielding mask as shown in FIG. 1A accordingto the first embodiment of the present invention;

FIGS. 3A and 3B show photographs of a magnified plan view of a state ofa crystal growing in the same direction as a temperature gradientdirection (in a horizontal direction) according to the first embodimentof the present invention;

FIGS. 4A and 4B are schematic diagrams showing an intensity distributionof an energy beam obtained when an optical system having high resolutionor low resolution is used according to the first embodiment of thepresent invention;

FIG. 5 is a diagram explaining an intensity gradient of an energy beamobtained when an optical system having high resolution is used and amagnified photo showing a state of growth of a crystal of semiconductorthin film which has occurred in the intensity gradient according to thefirst embodiment of the present invention;

FIG. 6 is a diagram explaining an intensity gradient of an energy beamobtained when an optical system having resolution being different fromthat of the optical system shown in FIG. 5 is used and a magnified photoshowing a state of growth of a crystal of a semiconductor thin filmwhich has occurred in the intensity gradient according to the firstembodiment of the present invention;

FIG. 7 is a graph showing a relation between a length “Le” in which anintensity gradient of an energy beam occurs and an error in relativepositions relative to a starting point for crystallization in anlight-shielded region where light is intercepted by a light-shieldingmask according to the first embodiment of the present invention;

FIG. 8 is a diagram explaining a length “Le” corresponding to a slopeportion of an intensity gradient of an energy beam according to thefirst embodiment of the present invention;

FIG. 9 is a diagram showing an error in a relative position (degree ofmeandering) relative to a starting point for crystallization accordingto the first embodiment of the present invention;

FIG. 10 is a magnified plan photo showing a state of melt andre-crystallization of a silicon thin film obtained when a pitch of amask pattern is changed according to the first embodiment of the presentinvention;

FIG. 11 is a graph showing a relation between irradiation intensity ofan energy beam and a light-shielding width of a light-shielding patternby which excellent growth of a crystal by melt and re-crystallizationoccurs under irradiation intensity according to the first embodiment ofthe present invention;

FIG. 12 is a diagram showing configurations of one example of airradiation device preferably employed in the method for manufacturing asemiconductor thin film according to the first embodiment of the presentinvention;

FIG. 13 is a diagram showing configurations of one example of a pulselaser irradiation device being able to be used in the method formanufacturing the semiconductor film according to the first embodimentof the present invention;

FIG. 14 is a magnified plan photo showing a state of a crystal of asilicon thin film realized according to a second embodiment of thepresent invention;

FIGS. 15A to 15I, and FIG. 15I′ are process flow diagrams of the methodfor manufacturing a TFT according to a third embodiment of the presentinvention;

FIGS. 16A to 16G, and FIG. 16G′ are process flow diagrams of thecomparative (conventional) method for manufacturing a TFT; and

FIG. 17 is a diagram illustrating a general and typical structure of aconventional TFT.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Best modes of carrying out the present invention will be described infurther detail using various embodiments with reference to theaccompanying drawings.

A semiconductor thin film of the present invention ismanufactured/completed by irradiating an energy beam on a preformedsemiconductor thin film with direct-light-shielded regions(photo-masking regions) to melt and re-crystallize the preformedsemiconductor thin film with its direct-light-shielded regions asstarting points. More specifically, in the above method, by irradiatingan excimer laser with a wavelength of 308 nm, using a mask projectionmethod, on a silicon (Si) thin film, melt and re-crystallization is madeto occur in the direct-light-shielded region of the silicon thin filmbeing intercepted by a light-shielding pattern of the mask and growth ofa silicon crystal occurs in a direction of a local temperature gradientwith the portion where the melting and crystallization occurs as astarting point. The present invention is thus characterized in that theirradiation of the energy beam gives energy to the direct-light-shieldedregion in the silicon thin film so that melt and re-crystallizationoccurs with the direct-light-shielded region as the starting point andin that the local temperature gradient in the direct-light-interceptedportion is made to be 300° C./μm or more. In the present invention, theabove temperature gradient acts as a large driving force to cause thecrystal to grow from its starting point to a specified direction. Theabove melt and re-crystallization process causes a recrystal to growfrom the starting point for occurrence of melt and re-crystallization tothe direction of a temperature gradient and, as a result, crystal grainsbeing large in size and having a uniform direction of its growth areformed. The method for manufacturing a semiconductor thin film of thepresent invention can be preferably applied to manufacturing of, forexample, an amorphous silicon (a-Si) thin film, poly-silicon (poly-Si)thin film, or a like, which enables re-crystallization of the siliconthin film to occur so that the silicon thin film having crystal grainsbeing large in size and having grown in a specified direction isproduced.

First Embodiment A Method for Manufacturing Semiconductor Thin Film

FIGS. 1A and 1B are diagrams explaining a method for manufacturing asemiconductor thin film according to a first embodiment of the presentinvention. FIG. 1A is a plan view illustrating one example oflight-shielding elements employed in the present invention. FIG. 1B is aschematic cross-sectional view explaining melt and re-crystallizationoccurring in a semiconductor thin film irradiated with a beam employedin the present invention. FIG. 2 shows a photograph of a magnified planview of a growth state of a silicon crystal observed when melt andre-crystallization have been performed by using the light-shielding maskas shown in FIG. 1A. FIGS. 3A and 3B show photographs of a magnifiedplan view of a state of a crystal growing in the same direction as atemperature gradient direction (in a horizontal direction).

The light-shielding elements are placed between a beam source and anobject to be irradiated (in an upper portion of a semiconductor thinfilm 14 or above the semiconductor thin film 14) to intercept part of anenergy beam 13 to be applied to the semiconductor thin film 14. As thelight-shielding element, for example, a light-shielding mask formed on aglass substrate 15 (FIG. 1B) by patterning, or a like is employed.Moreover, as described later, in the case of manufacturing a TFT of thepresent invention, a gate electrode formed on a gate insulating filmformed on a semiconductor thin film functions as a light-shieldingelement.

As shown in FIG. 1A, on a light-shielding mask 11 are arranged, at aspecified pitch (interval) “P”, stripe-shaped mask patterns 12 eachhaving an aperture width “W” among them and each having alight-shielding width “L”. An experiment employed in the presentinvention shows that, since, by using an optical system (to be describedlater) with high resolution, the temperature gradient in adirect-light-shielded region 17 (hereinafter may be referred to aslight-shielded region 17) in the semiconductor thin film 14 is made tobe 300° C./μm or more and, as shown in FIG. 1B, FIG. 2 and FIGS. 3A and3B, a crystal was able to be grown by a large growth driving force in adirection of a local temperature gradient from a starting point 16 wheregrowth of a crystal starts to occur. As a result, even if thelight-shielding mask 11 having a narrow light-shielding width L, a largeaperture width W, and a wide pitch P was used, a crystal of asemiconductor thin film was able to be grown in a manner in which anentire region including the starting point 16 of the light-shieldedregion 17 having the narrow width to the aperture portion 18 having thelarge width was filled with the crystal. Moreover, arrows 19 shown inFIG. 1B indicate a direction in which the crystal grows from each of thestarting points 16 toward the adjacent light-shielded region 17. Eachwhite vertical line 20 shown in FIG. 2 and FIGS. 3A and 3B indicate aportion in which a crystal grown from the light-shielded regions 17being adjacent to one another struck one another and in which a crystalwas growing in one direction (in a horizontal direction in FIG. 1B) in amanner in which the growth was directed toward the light-shieldedregions 17 being adjacent to one another.

In the present invention, since the light-shielded region 17 in thesemiconductor thin film 14 is made to have a temperature gradient, thelight-shielding mask 11 having a P/L ratio (ratio of the pitch P to thelight-shielding width L) being 1 or more, for example, being 10 or morecan be also used. Even when a light-shielded region having a smalllight-shielded region width is formed on a semiconductor thin film byusing the light-shielding mask, a large temperature gradient occurs inthe light-shielded region and, as a result, a crystal of thesemiconductor thin film can be grown in a direction of the temperaturegradient. Moreover, the P/L ratio has its specified upper limit which isdetermined depending on intensity of an energy beam, resolution of anoptical system, cooling state, or a like.

FIG. 4A is a schematic diagram showing an intensity distribution 23 ofan energy beam 13 obtained when an optical system 21 having highresolution is used according to the present invention. FIG. 4B is aschematic diagram showing an intensity distribution 23 of an energy beam13 obtained when an optical system 22 having low resolution is usedaccording to the present invention. FIGS. 5 and 6 are diagramsexplaining an intensity gradient of an energy beam obtained when opticalsystems each having different resolution are used and magnified photosshowing states of growth of a crystal of semiconductor thin film whichhas occurred by the intensity gradient according to the presentinvention.

As the energy beam 13 that can be employed in the present invention,various energy beams each having specified output intensity can be used.For example, an excimer laser with a wavelength of 308 nm which has anoutput characteristic of 100 mJ/cm² to 1000 mJ/cm² can be preferablyused. Moreover, the output characteristic of the energy beam is aparameter that can be varied depending on a diameter of a beam.

The energy beam 13 is applied by the optical system 21 with a resolutionof 4 μm or less as shown in FIG. 4A to a surface of the semiconductorthin film 14. The resolution of the optical system can be calculated byusing the equation: Resolution=(constant) x wavelength/number ofapertures. Preferably, the optical system 21 is used which has thenumber of apertures (NA) of 0.2 using an excimer laser with a wavelengthof 308 nm and has high resolution obtained by substituting a constantwithin 0.50 to 0.75 into the above equation. Moreover, the number ofapertures (NA) is given by a following equation: NA=n sin θ, where “n”denotes a refractive index of a medium. Since n=1 in the air, NA=sin θ.

In the present invention, the energy beam 13, part of which isintercepted by the light-shielding element, diffracts (turns around) thelight-intercepted portion 17 to heat the semiconductor thin film 14 and,by using the optical system 21 with such the high resolution, thelight-shielded region 17 in the semiconductor thin film 14 can be madeto have an intensity gradient of an energy beam of 220 mJ/cm²/μm ormore, for example. That is, by irradiating the semiconductor thin film14 with an energy beam 13 using the high-resolution optical system 21with a resolution of 4 μm or less, the light-shielded region 17 in thesemiconductor thin film 14 can be made to have the above intensitygradient and, as a result, a temperature gradient of 300° C./μm or morecan be realized. Moreover, an intensity gradient of the energy beam canbe evaluated by irradiating a fluorescent plate with a beam andmagnifying fluorescent light emitted from the fluorescent plate using alens and then reading it by a CCD (Charge Coupled Device).

On the other hand, in the case of the optical system 22 as shown in FIG.4B which has a resolution of about 10 μm or more, an intensity gradientof an energy beam is small and, as a result, unlike in the case of thepresent invention, it is impossible to realize a temperature gradient of300° C./μm or more. That is, the optical system 22, as shown in FIG. 4B,cannot provide a driving force enough to cause a crystal to grow in aspecified direction.

An intensity gradient of an energy beam is explained by using a concreteexample. For example, when the conventional optical system 22 with lowresolution (for example, the resolution of 10 μm or more) as shown inFIG. 4B is employed and a light-shielding mask having a lightintercepting width L of about 2 μm to 3 μm is used and intensity (calledas crystallization intensity) of an energy beam which is used tocrystallize a semiconductor thin film with the light-shielded region 17as a starting point is 180 mJ/cm² and intensity of an energy beam(called as irradiation intensity) in the aperture portion 18 is 400mJ/cm², an intensity gradient of the energy beam becomes about 73.3(=(400−180)/3) mJ/cm²/μm to 110 (=(400−180)/2) mJ/cm²/μm. Unlike in thecase of using the conventional optical system 22, when the opticalsystem 21 with high resolution (for example, a resolution of 4 μm less)as shown in FIG. 4A is employed and a light-shielding mask having alight intercepting width L of 1 μm or less is used and, as shown in FIG.5, intensity (called crystallization intensity) of an energy beam whichis used to crystallize a semiconductor thin film with the light-shieldedregion 17 as a starting point 16 is 180 mJ/cm² and intensity (calledirradiation intensity) of an energy beam in the aperture portion 18 is400 mJ/cm², an intensity gradient of the energy beam becomes 220(=(400−180)/1) mJ/cm²/μm or more.

Thus, according to the present invention, such the large intensitygradient of an energy beam as described above is realized by the opticalsystem with high resolution and, as a result, a temperature gradient inthe light-shielded region becomes large, which enables the temperaturegradient of 300° C./μm or more to be achieved. On the other hand, theconventional low-resolution optical system provides a small intensitygradient of an energy beam, which makes it impossible to increase thetemperature gradient in the light-shielded region. Due to this, in thepresent invention, even if a width of a light intercepting pattern (maskpattern 12) is made narrow and the light-shielded region is made small,the intercepting portion can be made to have a temperature gradientnecessary to cause a crystal to grow in a specified direction.

An upper limit of the temperature gradient of an energy beam may be setat 20000° C./μm, preferably at 13300° C./μm. The upper limit of thetemperature gradient is set by taking into consideration an abrasion ina region corresponding to an aperture portion caused by excess energyand micro-crystallization in a region corresponding to a light-shieldedregion. That is, a difference (T_(H)−T_(L)) between a maximumtemperature T_(H) at which an abrasion does not occur in an apertureportion where an energy beam directly strikes a semiconductor thin film(for example, in the case of silicon, its boiling point being 3267° C.is an upper temperature) and a temperature T_(L) which becomes astarting point for melt and re-crystallization in a light-shieldedregion where an energy beam is intercepted (for example, in the case ofa-Si, its melting point being 1150° C. is a lower limit temperature andin the case of poly-Si, its melting point being 1410° C. is a lowerlimit temperature) is an upper limit of the temperature gradient.Therefore, according to the present invention, in the case of 0. 1 μmwhich is the best resolution to be applied when a high-resolutionoptical system is used, the upper limit of a temperature gradient isabout 20000° C./μm [(T_(H)−T_(L))/resolution=(3267−1150)/0.1]. Moreover,results from experiments shown in FIG. 2 and FIGS. 7 and 10 (describedlater) show that it is preferable that a high-resolution optical systemwith a resolution of 0.15 μm is used so that a center position of 0.15μm being a half of a light-shielding width L being 0.3 μm becomes astarting point for melt and re-crystallization. In that case, an upperlimit of a temperature gradient is preferably about 13300° C./μm beingtwo thirds of 20000° C./μm being the temperature gradient describedabove.

Moreover, the “abrasion” denotes a phenomenon in which a semiconductorthin film melts, boils, and evaporates. The micro-crystallization (oramorphization) occurs after transition from equilibrium tonon-equilibrium caused by rapid cooling. Therefore, to preventoccurrence of micro-crystallization in a region corresponding to alight-shielded region, it is desirable that solidification of asemiconductor thin film starts with the semiconductor thin film kept inequilibrium thermodynamically. For the boiling point and melting pointdescribed above, refer to “Semiconductors and semimetals”, Vol. 23,Pulsed laser processing of semiconductors, Edited by R. F. Wood, C, W,White and R. T. Young, Academic Press, Inc., Orland, 1984.

According to the experiment described above, when, by applying a laserhaving energy intensity of 400 mJ/cm² to an a-Si thin film with athickness of, for example, 60 nm, the a-Si thin film was melt andre-crystallized using an optical system with a resolution of 1 μm, thetemperature gradient was [2428° C. (temperature reached by irradiationof the laser having the energy intensity of 400 mJ/cm²)−1150° C.(melting point of a-Si)]/1 μm=about 1200° C./μm. Moreover, when, byapplying a laser having the energy intensity of 600 mJ/cm² to the a-Sithin film with the thickness of, for example, 60 nm, the a-Si thin filmwas melt and re-crystallized using an optical system with a resolutionof 1 μm, the temperature gradient was [3267° C. (temperature reached byirradiation of the laser having the energy intensity of 600mJ/cm²)−1150° C. (melting point of a-Si)]/1 μm=about 2100° C./μm. Thisresult shows that the preferable temperature gradient ranges between1200° C./μm and 2100° C. /μm.

When a cost and life of an optical system to be used are taken intoconsideration, it is desirable that both resolution of the opticalsystem and irradiation intensity of a laser to be used are made low tosome extent. To achieve, at least, the objects of the present invention,from a viewpoint of the cost and life of the optical system, it ispreferable that an optical system with a resolution of, for example, 4μm is used and a laser having energy intensity of, for example, 400mJ/cm² is applied to melt and re-crystallize a semiconductor film.

Also, according to another experiment, when, by applying a laser havingenergy intensity of 400 mJ/cm² to an a-Si thin film with a thickness of,for example, 60 nm, the a-Si thin film was melt and re-crystallizedusing an optical system with a resolution of 4 μm, the temperaturegradient was [2428° C. (temperature reached by irradiation of the laserhaving the energy intensity of 400 mJ/cm²)−1150° C. (melting point ofa-Si)]/4 μm=about 300° C./μm. Moreover, when, by applying a laser havingthe energy intensity of 400 mJ/cm² to an a-Si thin film with a thicknessof, for example, 60 nm, the a-Si thin film was melt and re-crystallizedusing an optical system with a resolution of 1 μm, the temperaturegradient was [2428° C. (temperature reached by irradiation of the laserhaving the energy intensity of 400 mJ/cm²)−1150° C. (melting point ofa-Si)]/1 μm=about 1200° C./μm. This result shows that the preferabletemperature gradient ranges between 300° C./μm and 1200° C./μm.

It is desirable that energy of a laser sufficiently enough to cause, atleast, melt and re-crystallization to occur with a light-shielded regionas a starting point is applied to the light-shielded region. Accordingto another experiment, it was also confirmed that, when an a-Si thinfilm with a thickness of, for example, 60 μm was irradiated with anexcimer laser with a wavelength of 308 μm and with energy intensity of400 mJ/cm², with light being intercepted by a light-shielding maskhaving a light-shielding width of 1 μm and using an optical system witha resolution of 1 μm, energy intensity of 180 mJ/cm² was applied to alight-shielded region and, as a result, a temperature of thelight-shielded region exceeded 1150° C. being a melting point of a-Si,and melt and re-crystallization of the silicon thin film occurred withthe light interception portion as a starting point. Moreover, at thispoint, since the energy intensity of 400 mJ/cm² is applied to anaperture portion, a temperature gradient in the light-shielded regionbecame about 220° C./μm and, at the starting point, growth of a crystalin a direction along the temperature gradient was observed (see FIG. 5).As a result, as shown in FIG. 5, a demarcation line at the startingpoint became a clear straight line or approximately a straight line.

On the other hand, according to another experiment, when a silicon thinfilm with a thickness of, for example, 60 μm was irradiated with anexcimer laser with a wavelength of 308 nm and with energy intensity of400 mJ/cm², with light being intercepted by a light-shielding maskhaving a light-shielding width of 3 μm and using an optical system witha resolution of 10 μm, energy of 180 mJ/cm² was applied to thelight-shielded region and, as a result, a temperature of thelight-shielded region exceeded 1150° C. being the melting point of a-Si,and melt and re-crystallization of the silicon thin film occurred withthe light interception portion as a starting point. At this point,though energy of 400 mJ/cm² was applied to the aperture portion, sincethe temperature gradient in the light-shielded region was as gentle asabout 73.3° C./μm and, at the starting point, growth of the crystal in adirection other than the above direction (upper and downward directionin FIG. 6) tended to occur as well, the demarcation line at the startingpoint was not a straight line (see FIG. 6).

Since such the temperature gradient as described above is made in thelight-shielded region, the growth of the crystal in a direction beingvertical to a direction of the temperature gradient (left and rightdirections in FIG. 4) is suppressed and growth of the crystal in thedirection of the temperature gradient is facilitated.

FIG. 7 is a diagram showing a relation between a length “Le” in which anintensity gradient of an energy beam occurs and an error in a relativeposition (degree of meandering) relative to the starting point 16 forcrystallization in the light-intercepted portion 17 where light isintercepted by the light-shielding mask 11 shown in FIG. 1. Moreover,FIG. 8 is a diagram explaining a length “Le” of a slope portionindicating an intensity gradient of an energy beam. FIG. 9 is a diagramshowing an error in a relative position (degree of meandering) relativeto the starting point 16 for crystallization. The length “Le” of a slopeportion indicating an intensity gradient is calculated from a result ofmeasurement of an intensity gradient of the energy beam described aboveand, as a result of taking into consideration variations in intensity,in a fine region, of raw laser ray being unable to be corrected by anoptical uniforming unit, variations in performance of an opticaluniforming unit, or a like, the length within an intensity range from0.10 to 0.90 obtained when a beam intensity is set at 1.00 is defined asa length “Le” of a slope portion. Moreover, when the starting point 16is recognized, by noting a structural change in a crystal, a position inwhich a fine grain region and coarse and a big crystal are in contactwith each other is used as a starting point and variations (errors) inthe position of the starting point are expressed numerically

The result shown in FIG. 7 shows that, in order to effectively reduce anerror in a position of a starting point, Le≦4 μm. Thus, according to thepresent invention, based on the above result, an optical system withresolution being so high that a length “Le” in which an intensitygradient of an energy beam occurs is 4 μm or less, more preferably 1 μmor less in the light-shielded region, is preferably used and, as aresult, a temperature gradient is made large and the crystal can be madeto grow in a direction of the temperature gradient.

Next, arrangement pitches (P) of the stripe-shaped mask patterns in thelight-shielding mask are explained. FIG. 10 shows a state of melt andre-crystallization of a silicon thin film occurred when alight-shielding width L of the stripe-shaped mask pattern formed in thelight-shielding mask is set at 0.3 μm and a pitch of the mask pattern ischanged between 1.5 μm and 3.5 μm. At this point, a irradiationintensity of an energy beam was set at 578 mJ/cm².

As shown in FIG. 9, it was confirmed from the experiment that, when thepitch was between 2.5 μm to 3.5 μm, growth of the crystal in a directionof an intensity gradient of an energy beam occurred. However, when thepitch was 1.5 μm, no growth of the crystal in a horizontal direction wasobserved. The reason seems to be that, due to large amounts of energysupplied to the silicon thin film and additionally due to thermaldiffusion in a horizontal direction, a temperature in a region wherelight was to be intercepted rose excessively. When the pitch was 3.0 μm,the irradiated region was filled with crystals having grown in ahorizontal direction from adjacent light-shielded regions and desirablegrowth of the crystals, in particular, was observed. When the pitch was3.5 μm, a phenomenon in which a clearance among crystals having grownfrom the adjacent starting points in a horizontal direction wasobserved. This phenomenon occurs due to the reason that a nucleus almostspontaneously and randomly occurred by being cooled by thermalconduction to the substrate before crystals obtained by horizontalepitaxial growth with the starting point of the adjacent light-shieldedregion as a nucleus were in contact with one another and, as a result,polycrystalline films (micro-crystal film or amorphous crystal film)having crystal grains being relatively small in size were formed. Whenthe pitch was 2.5 μm, the epitaxial growth in the horizontal directionprogressed and terminated when crystals obtained by the epitaxial growthwere in contact with one another. However, as in the case where thepitch was 1.5 μm, there existed partially some regions in which, due tolarge amounts of energy supplied to the semiconductor thin film andadditionally due to thermal diffusion in a horizontal direction, atemperature in the region where light was to be intercepted roseexcessively. This phenomenon seems to have occurred due tonon-uniformity being inherent in the intensity of a irradiation beam.

These results show that it is desirable that the arrangement intervalamong the light-shielded regions is shorter than growth distancedetermined depending on a parameter such as an intensity and, moreparticularly, a relation between a pitch P of the light-shieldingpattern and the light-shielding width L is given by an expression of“pitch P/light-shielding width L>5 (=1.5/0.3)” and preferably by anexpression of “pitch P/light-shielding width L=10 (>1.5/0.3)”. From aviewpoint that a silicon thin film having crystal grains being large insize is obtained, after the above conditions are met, arrangementintervals of the light-shielded region are made preferably as long aspossible.

An upper limit of a value calculated by the expression of “a pitch P/alight-shielding width L” is set within a range in which a clearance doesnot occur among crystals having grown in a horizontal direction from anadjacent starting point. Main parameters include a value obtained by theexpression of “a pitch P/a light-shielding width L”, irradiationintensity of an energy beam, a thickness of a film, a waveform of a beampulse, or a like. By calibrating these parameters, the problem ofoccurrence of the clearance can be solved.

FIG. 11 is a graph of a result from an experiment showing a relationbetween irradiation intensity of an energy beam of 400 mJ/cm² to 900mJ/cm² and the light-shielding width L of a light-shielding pattern bywhich excellent growth of a crystal by melt and re-crystallizationoccurs under the irradiation intensity according to the presentinvention.

The light-shielding width L of a light-shielding pattern is inproportion to irradiation intensity of an energy beam. Therefore,according to the experiment, when the irradiation intensity was madelarge, in order to cause growth of a crystal to occur by excellent meltand re-crystallization under the irradiation intensity, it was necessaryto make large the light-shielding width L. Moreover, when a thickness ofthe semiconductor thin film was increased from 60 nm to 75 nm, byshifting the irradiation intensity to a side of high intensity, growthof a crystal by excellent melt and re-crystallization was made possible.

From the above result, the relation between the irradiation intensityand the light-shielding width L, which can cause growth of a crystal byexcellent melt and re-crystallization to occur, is expressed by anequation of “L (μm)=a×E (mJ/cm²)+b”, where “a” and “b” denotecoefficients which vary depending on a thickness of a semiconductor thinfilm being an object to be irradiated.

The light-shielding width L, as is apparent from FIG. 11, is setaccording to the irradiation intensity, thickness of the semiconductorthin film, or a like and is preferably 0.2 μm to 0.5 μm and morepreferably 0.3 μm to 0.5 μm. Here, when the light-shielding width “L”was less than 0.2 μm, no effect of light-shielded region was obtainedand micro-crystallization occurred on an entire of the semiconductorthin film and no growth of the silicon crystal in a horizontal directionwas observed. Also, if the light-shielding width “L” exceeded, forexample, 0.5 μm, though horizontal growth of the crystal occurred,amorphous layer resided in a center of light-shielded region.

Moreover, energy is given to the light-shielded region so that melt andre-crystallization occurs with the light-shielded region as a startingpoint. The energy to be given is different depending on a kind ofsemiconductor thin film and/or its thickness, however, if the thin filmis an a-Si thin film with a thickness of 75 nm formed on a glasssubstrate, its energy intensity is preferably within a range of 170mJ/cm² to 200 mJ/cm². In the light-shielded region where such the energyis given, a nucleus for melt and re-crystallization is produced and acrystal grows with the nucleus as a starting point. For example, asshown in FIG. 1B, since the stripe-shaped light-shielding pattern havingthe light-shielding width L is formed, the crystal of the semiconductorthin film grows in a direction (left and right directions in FIG. 1B) inwhich a temperature gradient occurs with the produced nucleus as thestarting point. At this point, it is preferable that the light-shieldingregions are arranged periodically at appropriate pitches and, forexample, as shown in FIG. 2, the semiconductor thin film having grown ina horizontal direction can be obtained.

Moreover, according to the present invention, in order to induceexcellent growth of the crystal, for example, the semiconductor thinfilm having been melt by irradiation with an energy beam is cooled at aspeed that does not cause micro-crystallization. Since it was found thatmicro-crystallization in an a-Si thin film with a thickness of 60 nmoccurs when being cooled at a speed of about 1.6×10^(10°) C./sec ormore, by controlling the cooling speed so as to be smaller than thespeed of 1.6×10^(10°) C./sec, micro-crystallization and amorphization inthe a-Si thin film can be prevented and a process of excellent growth ofthe crystal can be achieved.

In the semiconductor thin film manufactured by the methods describedabove, that is, in the semiconductor thin film having been melted andre-crystallized, a thickness of a starting point portion from whichgrowth of the crystal of the semiconductor thin film having been meltedand re-crystallized starts is smaller than a thickness of a terminatingportion of growth of the crystal and growth of its crystal occurs in adirection of the thickness gradient. For example, when an a-Si thin filmwith a thickness of 75 nm was melted and re-crystallized by irradiationwith an excimer laser with a wavelength of 308 nm having energyintensity of 820 mJ/cm² by the same method as employed by the melt andre-crystallizing unit, a film thickness of the starting point portion inwhich the melt and re-crystallization start was about 60 nm and athickness of a terminating portion of growth of the crystal was about100 nm. Such the phenomenon as described above, that is, the phenomenonin which the thickness of the starting point portion in which the meltand re-crystallization start is smaller than the thickness of aterminating portion of growth of the crystal and growth of the crystaloccurs in a direction of a thickness gradient is one that occursregardless of the initial film thickness, is a specific mode of themethod of manufacturing of the present invention. This phenomenon wasconfirmed even in the case in which the crystal of the abovesemiconductor thin film grew from its starting point for the growth, atleast, in two directions.

The manufacturing equipment of the present invention, to be describedlater (and above), employed in the method for manufacturing thesemiconductor thin film and the TFT of the present invention has a laserirradiation device which can cause, by applying an energy beam to thelight-shielding elements each being arranged between the semiconductorthin film and an energy beam radiating source, the crystal to grow in adesired direction with the light-shielded region of the semiconductorthin film as a starting point. Moreover, in the manufacturing equipmentfor fabricating a thin film transistor described later, an energy beamis applied by using gate electrodes each being arranged between asemiconductor thin film covered with a gate insulating film and anenergy beam irradiation source as a light-shielding element.

The above manufacturing equipment employed in the method formanufacturing the semiconductor thin film has the irradiation deviceequipped with an optical system with a resolution of 4 μm or less toachieve the above growth of the crystal. That is, since the irradiationdevice has the optical system with a resolution of 4 μm or less, asdescribed above, it is made possible to give a local temperaturegradient which enables the crystal to grow in a desired direction withthe light-shielded region as the starting point to the light-shieldedregion in which light is intercepted by the light-shielding element (orthe gate electrode).

Also, a light-shielding pattern constructed by forming thelight-shielding pattern on a transparent substrate is preferablyemployed as the light-shielding element to be employed in the aboveirradiation device in terms of convenience and a ratio (P/L) of alight-shielding width L of the light-shielding pattern to a pitch P forthe light-shielding pattern is preferably 1 (one) or more. Thelight-shielding width L of the light-shielding pattern is preferably 0.3μm or more. The reasons for this have been already described above anddescription of it is omitted accordingly. Moreover, in the case of theTFT, the gate electrode constructed by being formed with the gateinsulating film being interposed between the gate electrode and thesemiconductor thin film acts as the light-shielding element and itslight-shielding width L is a width of the gate electrode.

According to the present invention, in order to achieve more effectiveprocesses of melt and re-crystallization of semiconductor thin films,the irradiation device is equipped with a projection exposing unitadapted to simultaneously melt and re-crystallize entire surfaces of thesemiconductor thin films by applying an energy beam one pulse. Theprojection exposing unit that can be employed in the present inventionmay be selected from various types of projection exposing unitsdepending on kinds or energy intensity of an energy beam to be used, anarea of an object to be irradiated in which growth of the crystal isexpected. For example, a reduction projection optical system, 1:1projection optical system, or expansion projection optical system can beattached to the irradiation device to construct the manufacturingequipment of the present invention.

FIGS. 12 and 13 show one example of irradiation devices being applicableto manufacturing of a semiconductor thin film and TFT of the presentinvention. FIG. 12 is a diagram showing configurations of one example ofa irradiation device preferably employed in the method for manufacturinga semiconductor thin film according to the present invention.

In the irradiation device as shown in FIG. 12, pulse UV (Ultra-violet)light supplied from a first excimer laser EL1 and a second excimer laserEL2 is introduced via mirrors opt3 and a lens opt4 into a homogenizeropt20. An intensity profile of the UV light beam is shaped by thehomogenizer opt20 so that it has a desired uniformity, for example, itsdistribution within a surface becomes ±5% at the optical mask opt21.

Moreover, there are some cases in which an intensity profile or a totalenergy amount of energy of the beam supplied originally from the excimerlasers varies among pulses, the irradiation device preferably has amechanism to uniform spatial distribution of its intensity and/orvariations in its intensity among pulses on an optical mask. As thehomogenizer opt20, a flyeye lens or a cylindrical lens is generallyused.

An optical pattern formed by the above optical mask is applied via areduction projection exposure system opt23 and a laser introducingwindow W0 to a substrate sub0 placed within a vacuum chamber C0. Thesubstrate sub0 is placed on a substrate stage S0 and the optical patternis exposed, by operations of the substrate stage S0, in a desired regionon the substrate sub0, for example, on a pattern transfer region ex0.Though an example in which the reduction projection exposure systemopt23 is used is shown in FIG. 12, a 1:1 projection optical system orexpansion projection optical system may be employed depending on asituation.

By movement of the substrate stage S0 (directions X-Y in FIG. 12), anarbitrary region on the substrate sub0 is irradiated with a laser. Also,the optical mask on which the light-shielding pattern is formed isplaced on a mask stage (not shown). The beam applied to the substratesub0 is controlled by movement of the optical mask within an exposureenabled region.

Next, a mechanism adapted to apply an optical pattern to the substratesub0 under a desired condition is described. Since calibration of anoptical axis requires delicate calibrating operations, a method in whichan optical axis having already completed the calibration is fixed toadjust a position of the substrate sub0 is shown in descriptions below.

A position of a substrate irradiation surface relative to an opticalaxis is corrected by a position of a focus alignment direction Z andverticality relative to an optical axis. More specifically, out of atilt correcting direction θxy, tilt correcting direction θxz, tiltcorrecting direction θyz, exposure region movement direction X, exposureregion movement direction Y, and the focus alignment direction Z, bycalibrating the tilt correcting direction θxy, tilt correcting directionθxz, and tilt correcting direction θyz, the verticality relative to theoptical axis is corrected. Moreover, the substrate irradiation surfaceis controlled to be arranged in a position corresponding to a focusdepth of an optical system by calibrating the focus alignment directionZ.

In the TFT being formed on an insulator in particular, since a glasssubstrate being inferior to a silicon wafer in surface accuracy is usedas the insulator, such the irradiation device as is equipped with acorrecting mechanism described above is required to be effective inachieving good growth of the crystal of the semiconductor thin film.

Moreover, a pulse laser irradiation device as shown in FIG. 13 may beused in the manufacture of the semiconductor thin film or TFT of thepresent invention.

In the pulse laser irradiation device shown in FIG. 13, laser ray beingsupplied from a pulse laser source 1101 reaches a silicon thin film 1107being an object to be irradiated through an optical path 116 defined byan optical device group including mirrors 1102, 1103, and 1105 and abeam homogenizer 1104 on a glass substrate 1109. An aim of the beamhomogenizer 1104 is to uniform spatial intensity of the beam to beapplied. In such the irradiation device as above, laser irradiation toan arbitrary position on the substrate is performed by moving the glasssubstrate 1109 on an xy-stage 1108. The laser irradiation to anarbitrary position on the substrate may be performed not only by movingthe xy stage but also by moving the optical element group describedabove or by a method in which movement of the optical element group iscombined with that of the stage. For example, the laser irradiation maybe performed by placing the substrate on an x-direction stage and thehomogenizer on a y-direction stage. Moreover, the laser irradiation maybe performed in a vacuum within a vacuum chamber or in a high-purity gasenvironment. Moreover, the laser irradiation device may be equipped witha glass-substrate containing cassette 1110 in which a silicon thin filmis formed and a substrate transporting mechanism 1111, as needed. Thesubstrate may be mechanically put and withdrawn between the cassette andthe stage.

According to the present invention, since the manufacturing equipmenthaving such the irradiation device as described above, melt andre-crystallization of a semiconductor thin film can be efficientlyrealized. A remarkable effect of the present invention is that melt andre-crystallization can be achieved by one pulse irradiation with anenergy beam even if the semiconductor thin film has a large area inparticular.

Second Embodiment Another Method for Manufacturing Semiconductor ThinFilm

Next, a method for manufacturing semiconductor thin film according to asecond embodiment of the present invention will be described. Byemploying the method for manufacturing a semiconductor thin film of thepresent invention described above, as shown in FIG. 14, a siliconcrystal could be grown radially by melt and re-crystallization.

For example, the silicon crystal shown in FIG. 14 is obtained by usingthe light-shielding mask in which dot-shaped light-shielding patternseach having a diameter of 1.5 μm are placed at an equal interval pitchof about 4 μm and applying an excimer laser with a wavelength of 308 nmand with an energy intensity of 467 mJ/cm² to a silicon thin layer witha thickness of 60μm. As is understood from FIG. 14, the silicon crystalgrows radially from the light-shielded region and no clearance isobserved between the silicon crystal having grown from thelight-shielded region and the silicon crystal having grown from adjacentlight-shielded regions.

Third Embodiment A Method for Manufacturing TFT

Next, a method for manufacturing TFT according to a third embodiment ofthe present invention will be described. The method for manufacturing asemiconductor thin film described above is applied to manufacturing of aTFT according to the embodiment. An example of the method formanufacturing a TFT that can obtain a crystallized film in a desiredregion in a self-aligning manner is explained by referring to a processflow diagram shown in FIGS. 15A to 15I, and FIG. 15I′.

That is, in the method for manufacturing a TFT of the present invention,a crystallized film whose crystal grows in one direction with alight-shielded region of a semiconductor thin film as a starting pointis formed by using a gate electrode formed on the semiconductor thinfilm with a gate insulating film being interposed between the gateelectrode and the semiconductor thin film as a light-shielding elementand by applying an energy beam to the semiconductor thin film. Thus,this embodiment is characterized in that the irradiation of an energybeam can give energy to the light-shielded region so that it can act asthe starting point for melt and re-crystallization of the semiconductorthin film and so that a local temperature gradient in the light-shieldedregion is made to be 300° C./μm or more.

In the method for manufacturing a TFT according to this embodiment, byemploying the gate electrode formed on the gate insulating film as thelight-shielding element, a starting point for melt andre-crystallization is formed in the silicon thin film positioned in adownward direction of the gate electrode. Then, the silicon crystal ismade to grow in a self-aligning manner from the starting point in adirection of a width of the gate electrode. Since the direction ofgrowth of the silicon crystal matches the direction (source-drain) ofcarrier transport in a channel, the obtained silicon thin film becomesan active layer having a high characteristic of carrier transport. Themethod for manufacturing a TFT of the embodiment can be preferablyapplied in the case in which a length of a channel reaches a sub-micronlevel.

As shown in FIG. 15A, a substrate cover film T1 and a silicon thin filmT2 are sequentially formed on a glass substrate sub0 from which organicsubstances, metals, fine particles, or a like have been removed bycleaning.

A silicon oxide film with a thickness of 1 μm is formed as the substratecover film T1 using silane and oxygen gas as its materials by an LPCVD(Low Pressure Chemical Vapor Deposition) method under condition of 450°C. By using the LPCVD method, an entire outer surface of the substrateexcept a substrate holding region can be covered (not shown). Instead ofthe LPCVD method, a plasma CVD method using tetraethoxysilane (TEOS) andoxygen as its material, an atmospheric CVD method using TEOS and ozoneas its material, a remote plasma CVD method in which a deposition regionis separated from a plasma generating region, or a like may be used. Asthe substrate cover film T1, materials being able to prevent diffusionof an impurity, contained in a substrate material, harmful to asemiconductor device, such as glass with alkaline metal concentrationreduced to a minimum and quarts and glass whose surface is polished, ora like, are effectively used.

The silicon thin film T2 is formed by the LPCVD method using disilane asits material at 500° C. so that the silicon thin film T2 has a thicknessof 75 nm. At this point, since a hydrogen atom concentration containedin the silicon thin film T2 is 1 atom % or less, a film roughness and alike caused by a release of hydrogen in a process of laser irradiation(described later) can be prevented. Moreover, the silicon thin film T2can be formed also by using the plasma CVD method, and by adjusting asubstrate temperature, a flow rate of hydrogen to silane, a flow rate ofhydrogen to silane tetrafluoride, or a like, a silicon thin film havinga low hydrogen atom concentration can be formed.

Next, the above substrate, after gas in a thin film forming equipmenthas been exhausted, is transported to a plasma CVD chamber through asubstrate transport chamber. Then, in the plasma CVD chamber, as shownin FIG. 15B, a first gate insulating film T3 is deposited on the siliconthin film T2. The gate insulating film T3 is made up of a silicon oxidefilm with a thickness of 10 nm deposited at a substrate temperature of350° C. by using silane, helium, and oxygen as material gas. Thereafter,as necessary, hydrogen plasma processing and heat annealing processingare performed on the first gate insulating film T3. A sequence ofProcessing as described above is performed in the thin film formingequipment.

Next, as shown in FIG. 15C, a layer-stacked island made up of thesilicon thin film T2 and silicon oxide film (first gate insulating film)T3 is formed by using photolithography and etching technology. At thispoint, the etching is preferably performed on condition that an etchingrate of the silicon oxide film (first gate insulating film) T3 is higherthan that of the silicon thin film T2 and, as shown in FIG. 15C, byforming a cross section of the pattern so as to be in a stepped (ortapered) shape, gate leakage is prevented and a TFT with highreliability can be obtained.

Then, the substrate having undergone the etching processing is cleanedto remove organic substances, metals, fine particles, or a like and, asshown in FIG. 15D, a second gate insulating film T4 is formed in amanner in which the above island is covered. The second gate insulatingfilm T4 is made up of a silicon oxide film with a thickness of 30 nmdeposited at 450° C. by the LPCVD method using silane and oxygen gas asits materials. To form the second gate insulating film T4, the plasmaCVD method using tetraethoxysilane (TEOS) and oxygen as its material,the atmospheric CVD method, the plasma CVD method or a like using TEOSand ozone as its material, or a like can be used.

Next, on the second gate insulating film T4 are formed an n⁺ siliconfilm with a thickness of 80 nm as a gate n⁺ electrode and a tungstensilicide film with a thickness of 110 nm. It is desirable that acrsytalline phosphorus-doped silicon film formed by the plasma CVDmethod or LPCVD method is used as an n⁺ silicon film. Then, afterphotolithography and etching processes have been performed, as shown inFIG. 15E, a patterned gate electrode T5 b is formed.

Then, after cleaning to remove organic substances, metals, fineparticles, surface oxidized films, or a like has been done, thelayer-stacked film is put into the laser irradiation device to irradiatethe silicon thin film T2 with a laser ray L0 using the gate electrode T5b as a light-shielding mask, as shown in FIG. 15F. The laser irradiationdevice that can be used here includes the laser irradiation device shownin FIG. 12 and the pulse laser irradiation device shown in FIG. 13. Byirradiation with the laser ray L0, the silicon thin film T2 is changedto a crystallized silicon thin film T6. The laser crystallization isperformed in an atmosphere of nitrogen having a purity being as high as99.9999% or more at a pressure of 700 torr or more and, after the laserirradiation has been completed, oxygen gas is introduced. At this point,by designing a light-shielding width “L” and light-shielding interval“P” properly, crystallization of silicon at a irradiated surface can berealized.

Next, as shown in FIG. 15G and FIG. 15G′, an impurity is implanted,using the above gate electrode T5 b as a mask, into the crystallizedsilicon thin film T2 to form an impurity implanted regions T6 and T6′.Moreover, when a CMOS circuit is fabricated, an n-channel TFT requiringan n+region and p-channel TFT requiring a p⁺ region are formed in aseparated manner by using photo-lithography in combination. As themethod for implanting an impurity, ion doping in which mass separationof the impurity to be implanted is not performed, ion implanting, plasmadoping, laser doping, or a like can be employed. In the process ofimpurity implantation, the impurity is implanted with silicon oxidedeposited on the surface being left (see FIG. 15G) or with the siliconoxide deposited on the surface being removed (see FIG. 15G′), dependingon applications or impurity implanting methods.

Moreover, the process of implanting the impurity shown in FIG. 15G andFIG. 15G′ may be performed prior to the process of crystallization shownin FIG. 15F. At this point, at the same time when the lasercrystallization process is performed, activation of the impurityimplanted by the method described above can be performed.

Next, as shown in FIG. 15H, a gate metal electrode T5 a is formed, by apatterning operation, on the gate electrode T5 b. Then, as shown in FIG.15I and FIG. 15I′, after interlayer separation insulating films T7 andT7′ have been deposited, a contact hole is formed and a metal film forwiring is deposited. Furthermore, a metal wiring T8 is formed by usingphotolithography and etching technology. As the interlayer separationinsulating films T7 and T7′, a TEOS oxide film, silica coating film, ororganic coating film, all of which can achieve planarization of a film,can be employed. The process of forming the contact hole is performed byusing photolithography and etching technology. As the metal wiring T8,aluminum of low resistance, copper, aluminum or copper-based alloy, ametal of high melting point such as tungsten, molybdenum, or a like canbe used.

By performing such the processes as above, a TFT having high performanceand high reliability of the present invention can be formed.

In the TFT manufactured as above, it was confirmed by experiments that,as in the case of the phenomenon described in manufacturing the abovesemiconductor thin film, a thickness at a starting point from whichgrowth of the silicon crystal of the melted and re-crystallized siliconthin film T6 started was smaller than that at a terminating point ofgrowth of the silicon crystal and the growth of the silicon crystaloccurred in a direction of a thickness gradient.

Fourth Embodiment Another Method for Manufacturing TFT

Next, a method for manufacturing TFT according to a fourth embodiment ofthe present invention will be described. A case in which an alignmentmark is assigned in advance and a laser is irradiated according to thealignment mark or a case in which the alignment mark is produced at thesame when a laser is irradiated is described. In these cases, the fourthembodiment of the method for manufacturing a TFT differs from the thirdembodiment of the method for manufacturing the TFT in following points.

That is, in the case where the alignment mark is assigned in advance anda laser is irradiated according to the alignment mark, a substrate coverfilm T1 and the tungsten-silicide film are sequentially formed on theglass substrate sub0 from which organic substances, metals, fineparticles or a like have been removed by cleaning. Next, in order toform the alignment mark on the substrate, the tungsten silicide filmpatterned by photo-lithography and etching is formed. Also, in order toprotect the alignment mark, a mark protecting film is formed and then asilicon thin film is formed.

In the process thereafter, when exposure is performed using the laserray, a desired region is exposed by using the alignment mark as apositional reference. Then, an alignment process is performed by usingthe alignment mark assigned in advance or an alignment mark (not shown)formed by patterning a crystallized silicon thin film as a positionalreference.

According to this embodiment, since a process of precisely selecting acrystallized region using a mask projection method is not required, evenif a transistor is further miniaturized, crystallization by a laserprocessing equipment using the same alignment method as the conventionalmethod is made possible and, therefore, time required for the alignmentprocess can be shortened and an increase in costs for the equipment canbe suppressed.

Comparative Method for Manufacturing TFT

An example of a comparative (conventional) method for manufacturing aTFT (as disclosed in Japanese Patent Application Laid-open No.2001-28440) is described by referring to a process flow diagram shown inFIGS. 16A to 16G, and FIG. 16G′. Moreover, since materials for thinfilms and thin film forming methods employed in the comparative methodare the same as employed in the case of the first embodiment of themethod for manufacturing a TFT, points that differ from the above firstembodiment only are described below.

First, as shown in FIG. 16A, a substrate cover film T1 and a siliconthin film T2 are sequentially formed on a glass substrate sub0 that havebeen already cleaned.

Next, as shown in FIG. 16B, after having cleaned the glass substratesub0, the glass substrate sub0 is put into a thin film forming equipmenthaving the energy beam irradiation device. In the thin film formingequipment, laser ray L0 is applied to the silicon thin film T2 and thesilicon thin film T2 is changed to a crystallized silicon thin film T2.Such the laser crystallization of the silicon thin film is performed inan atmosphere of nitrogen having a purity being as high as 99.9999% ormore at a pressure of 700 torr or more. After the laser irradiation hasbeen completed, oxygen gas is introduced.

At this point, by designing a light-shielding width “L” andlight-shielding interval “P” properly, crystallization of silicon at airradiated surface can be realized.

Next, the substrate having undergone the above process, after gas in athin film forming equipment has been exhausted, is transported to aplasma CVD chamber through a substrate transport chamber. Then, in theplasma CVD chamber, as shown in FIG. 16C, a first gate insulating filmT3 (silicon oxide film with a thickness of 10 nm) is deposited on thecrystallized silicon thin film T2.

Then, as shown in FIG. 16D, a layer-stacked island made up of thecrystallized silicon thin film T2 and silicon oxide film T3 is formed byphotolithography and etching technology.

Next, after the substrate having undergone the etching process has beencleaned, a second gate insulating film T4 (silicon oxide film with athickness of 30 μm) is formed in a manner in which the island is coveredwith the second gate insulating film T4.

Then, on the second gate insulating film T4 are formed an n⁺ siliconfilm with a thickness of 80 nm and a tungsten silicide film with athickness of 110 nm. Then, after photolithography and etching processeshave been performed, as shown in FIG. 16E, a patterned gate electrode T5is formed.

Next, impurity implanted regions T6, T6 are formed by using the abovegate electrode T5 as a mask. Moreover, when a CMOS circuit isfabricated, an n-channel TFT requiring an n⁺ region and p-channel TFTrequiring a p⁺ region are formed in a separated manner by usingphotolithography in combination. In the process of impurityimplantation, the impurity is implanted with silicon oxide film formedon the surface remaining (see FIG. 16F) or with the silicon oxide filmformed on the surface being removed (see FIG. 16F′), depending onvarious applications or impurity implanting methods.

Next, as shown in FIG. 16G and FIG. 16G′, after interlayer separationinsulating films T7 and T7′ have been deposited, a contact hole isformed and then a metal film for wiring is deposited. Then, a metalwiring T8 is formed by photolithography and etching technology. Theprocess of forming the contact hole is performed by photolithography andetching technology.

By performing processes as above, the conventional TFT to be compared asa reference is completed.

It is apparent that the present invention is not limited to the aboveembodiments but may be changed and modified without departing from thescope and spirit of the invention.

1. A semiconductor thin film manufactured by a process comprising thefollowing steps: causing a preformed semiconductor thin film to melt andre-crystallize with a light-shielded region therein as a starting pointof its melt and re-crystallization, by irradiating said preformedsemiconductor thin film with an energy beam partially intercepted by alight-shielding element; wherein irradiation of said energy beam givesenergy to said light-shielded region so that melt and re-crystallizationoccur with said light-shielded region as said starting point and so thata local temperature gradient in said light-shielded region is made to be300° C./μm or more; wherein a thickness of the starting point from whichgrowth of the crystal of said preformed semiconductor thin film havingbeen melted and re-crystallized starts is smaller than a thickness of aterminating portion of growth of the crystal and growth of the crystaloccurs in a direction of a thickness gradient.
 2. The semiconductor thinfilm according to claim 1, wherein the crystal of said preformedsemiconductor thin film grows from its starting point for the growth, atleast, in two directions.
 3. The semiconductor thin film according toclaim 1, wherein said light-shielding element is a light-shielding maskobtained by forming a light-shielding pattern on a transparentsubstrate.
 4. The semiconductor thin film according to claim 3, whereina ratio (P/L) between a light-shielding width L of said light-shieldingpattern and a pitch P of said light-shielding pattern is 1 (one) ormore.
 5. The semiconductor thin film according to claim 3, wherein thelight-shielding width L of said light-shielding pattern is 0.3 μm ormore.
 6. The semiconductor thin film according to claim 1, wherein saidpreformed semiconductor thin film before being melted andre-crystallized is made from an amorphous silicon or a poly-crystallinesilicon.
 7. A thin film transistor manufactured by a process comprisingthe following steps: forming a crystallized film by making a crystal ofa preformed semiconductor thin film grow in one direction with alight-shielded region in said preformed semiconductor thin film as astarting point by applying an energy beam to said preformedsemiconductor thin film using a gate electrode formed with a gateinsulating film interposed between said gate electrode and saidpreformed semiconductor thin film as an light-shielding element; whereinirradiation of said energy beam gives energy to said light-shieldedregion so that melt and re-crystallization occur with saidlight-shielded region as said starting point and so that a localtemperature gradient in said light-shielded region is made to be 300°C./μm or more; and wherein a thickness of the starting point from whichgrowth of the crystal of said preformed semiconductor thin film havingbeen melted and re-crystallized which makes up said thin film transistorstarts is smaller than a thickness of a terminating portion of growth ofthe crystal and growth of the crystal occurs in a direction of athickness gradient thereof.
 8. A semiconductor thin film, comprising: asemiconductor thin film layer comprising a light-shielded region and anaperture region, wherein the layer is preformed, melted andre-crystallized at a starting point within the light-shielded region,wherein said melting is caused by irradiating the layer with an energybeam that is partially intercepted by a light-shielding element so thata local temperature gradient in said light-shielded region is 300° C./μmor more; wherein a thickness of the starting point is smaller than athickness of a terminating portion of crystal growth and crystal growthoccurs in a direction of a thickness gradient.
 9. The semiconductor thinfilm according to claim 8, wherein a crystal of said semiconductor thinfilm grows from its starting point in at least two directions.
 10. Athin film transistor comprising: a crystallized film formed in apreformed semiconductor thin film, wherein crystals are grown in onedirection from a starting point in a light-shielded region of said thinfilm; a gate electrode; and a gate insulating film interposed betweensaid gate electrode and said preformed semiconductor thin film; whereinan energy beam melts said light-shielded region and re-crystallizationoccurs within said light-shielded region at said starting point, whereinsaid gate insulating film partially intercepts said energy beam so thata local temperature gradient in said light-shielded region is 300° C./μmor more; wherein a thickness of said thin film at the starting point issmaller than a thickness at a terminating portion crystal growth andcrystal growth occurs in a direction of a thickness gradient.